1. Field of the Invention
The present invention relates in general to plastic tubing and, more particularly, to apparatus and methods for making flexible and externally helically ribbed or beaded tubing integrally incorporating at least one electrically conductive wire.
2. Description of Related Art
Tubing having a relatively thin wall and an integral helical supporting bead is known in the art. Such support-bead tubing construction provides substantial crush resistance while leaving the tube wall flexible enough to permit short-radius bends without collapsing or kinking the tube. The versatility of this kind of tubing is evidenced by its wide applicability in construction, ventilation, manufacturing processes, auto washes, hospitals and other fields.
The walls of a support-bead tubing can be quite thin to minimize overall weight. This light weight for the tubing is an important feature, for example, in an inhalator tube to provide a patient with more comfort during oxygen delivery. Two other features of known thin wall support-bead or bead-reinforced tubing are transparency and smoothness of bore. Transparent plastic material permits inspection of the fluid coursing through the tube, to detect, for example, the presence of moisture in an anesthetic or patient oxygen delivery application. A smooth inner surface of such a tube is desirable to keep the tube free from deposits of contaminants and to discourage non-laminar flow.
U.S. Pat. No. 3,910,808 to Steward, assigned to the same assignee as this application, discloses apparatus for forming such thin-walled, flexible, crush resistant support-bead tubing. Steward discloses means for extruding a plastic strip having a longitudinal rib, and winding means for helically winding the strip about an axis to produce a corrugated flexible tubing having a smooth bore.
Many applications, however, require or are enhanced by the presence of controlled heating of such tubing. Neonatal patients, as well as patients in shock or who are sustained on breathing equipment, are among those who benefit from gas flowing through heat-conditioned tubing.
Prior attempts to achieve heating of a tube include providing: a resistance element extending linearly along the tube's axis (U.K. Patent No. 1,448,473 to Grant); fabric tape secured to a wire and applied to the tube (U.K. Patent No. 897,292 to Reik); or resistive wire in a tubing with no crush-resistant or thin-walled features (U.S. Pat. No. 4,038,519 to Foucras; U.S. Pat. No. 4,553,023 to Jameson et al.; and U.S. Pat. No. RE. 29,332 to Bilbro et al.). These tubings and those to be described in more detail in this section have one or more characteristics inconsistent with use in a medical environment. For example, tubing which relies on an adhesive binding for the support bead may deteriorate as a result of repeated sterilization. Materials may be biomedically incompatible, and exteriors are invariably characterized by crevices adjacent to the support bead which can harbor particulate matter and microbes. In addition, the more separate and distinct steps that are required in producing such tubing, the greater the cost, complexity and potential for failure of the product in use.
Another heatable tubing is described by DE 42 44 493A1 to Eilentropp. The '493A1 patent is believed to describe a respiratory tube with a spirally ribbed outer surface upon which electric heating conductors are placed adjacent to the spiral ribs. The heating conductors may be glued on to the outer surface of the tube. The ribs may be formed as a separate smaller tube profile which is then glued to the outer surface of the respiratory tube in a spiral arrangement. With a respiratory tube according to the '493A1 publication, the resistance wires must be separately secured to the outer surface of the tube, requiring a separate manufacturing step. Also, the separate glue may not provide as secure an attachment of the heating conductors to the respiratory tube as would be desired. There is a distinct possibility of imperfect match between the tube and the glue, and the glue also presents a possibility of solvents being released in the medical environment. As previously mentioned, the glue may not endure sterilization as well as the tube itself.
Finally, U.S. Pat. No. 3,686,354, issued to Makin, is believed to provide a thin-walled, flexible, but helically-ribbed collapse-resistant hose for inhalation apparatus. An inner thin-walled flexible tubular member defines a helical groove to which is helically secured an outer heater cable. The electrical heating cable is round in cross-section and is bonded to the outer surface of the tubular member by adhesive or vulcanization. With the inhalation hose according to the '354 patent, the helical heater cable does not become an integral part of the inner tubular member, but instead lays in a helical groove of the inner tubular member, defining a helical crevice on each side of the heater cable. This crevice or pair of crevices may provide an area in which soil and bacteria can escape cleaning and sterilizing efforts. Also, the heat originating at the conductors of the heater cable must be conducted through not only the insulation on this cable but also through to the wall of the inner tube. In fact, these heating conductors would appear to be more directly coupled to the ambient air than to tidal air in the tube.
No prior product, method of manufacture, or apparatus is known which provides a transparent, sterilizable, thin-walled, smooth bore tube having a resistive wire helically imbedded in the outer surface of the tube and fully protected by a contemporaneously wound supporting and encapsulating bead, the bead, wire and tube forming a unitary structure with a smooth, crevice-free outer surface.